Pneumonia caused by the opportunistic fungus Pneumocystis carinii is a major cause of morbidity and mortality in the immunocompromised host, including those with AIDS. The long-range goal of this proposal is to characterize the structure and function of proteins of importance in the biology of P. carinii and the pathogenesis of P. carinii Pneumonia (PCP), in order to identify potential targets for therapy against infection. We will focus on a newly described serine protease of mouse P. carinii, KEX1. The homology of mouse P. carinii KEX1 to the kexin family of fungal proteases and the mammalian prohormone convertases (furins) suggests it may be involved in the proteolytic processing and maturation of other P. carinii proteins, thereby playing an important role in the biology of P. carinii. A potential substrate for KEX1 is the adhesive surface antigen of P. carinii, glycoprotein A (gpA), which must be post-translationally processed to remove a conserved leader sequence (UCS) prior to placement on the organism's surface. A lysine-arginine dipeptide sequence recognized by the kexin family of proteases is found at the UCS-gpA junction in both animal and human P. carinii. The investigators will test the hypothesis that mouse P. carinii KEX1 can cleave the mouse P. carinii UCS-gpA junction. They will also characterize further the molecular biology and substrate specificity of KEX1. The specific aims of the proposal are: (1) To determine the level of KEX1 RNA and protein expression in different life cycle forms of mouse P. carinii; (2) To determine the subcellular location of KEX1 protein in mouse P. carinii; (3) To investigate functional aspects of mouse P. carinii KEX1; and (4) To characterize the genomic organization of the mouse P. carinii KEX1 gene. The proposed studies of mouse P. carinii KEX1 will provide a basis for comparison to similar enzymes in other members of the P. carinii family. Since all P. carinii synthesize protein species that must be post-translationally processed, an understanding of the structure and function of the kexin-like proteases of P. carinii may lead to the design of specific enzyme inhibitors that would adversely affect the biological processes of this important pathogen.